xbps-src: add default build option description

Most descriptions already just use this format anyways.

common/xbps-src/shutils/show.sh

@@ -129,15 +129,16 @@ show_pkg_makedepends() {
 }
 
 show_pkg_build_options() {
-    local f opt desc
+    local f
 
     [ -z "$PKG_BUILD_OPTIONS" ] && return 0
 
     source $XBPS_COMMONDIR/options.description
     msg_normal "$pkgver: the following build options are set:\n"
     for f in ${PKG_BUILD_OPTIONS}; do
-        opt="${f#\~}"
-        eval desc="\${desc_option_${opt}}"
+        local opt="${f#\~}"
+        local descref="desc_option_${opt}"
+        local desc="${!descref-Enable support for $opt}"
         if [[ ${f:0:1} == '~' ]]; then
             echo "   $opt: $desc (OFF)"
         else